1. Field of the Invention
The present invention is in the field of ceramic compositions used as a substrate for a thin metal film, the ceramic compositions having improved coefficients of thermal linear expansion which provides a minimum risk of detachment between the two.
2. Description of the Prior Art
In the field of magnetic recording, the tendency is toward higher density recording, and using higher frequency signals for recording. To achieve higher density recording, use has been made of metal powder tapes containing powders of ferromagnetic metals such as iron, cobalt, and nickel or so-called evaporated tapes which include ferromagnetic metal materials deposited by evaporation on a base film. To make use of such magnetic materials, the magnetic head used must have a high magnetic saturation flux density Bs.
In the new high density recording methods, the track width of the magnetic recording becomes very small. Accordingly, there is a corresponding need for an extremely narrow track width on the magnetic head.
The newer magnetic heads may be of the composite type wherein a non-magnetic substrate and a magnetic metal thin film to be used as the magnetic core are stacked one on the other and the magnetic thin film is used as the track. Another form of magnetic recording-reproducing head consists of one in which the thin film of magnetic metal and the conductive metal thin film are stacked on a non-magnetic base with intermediate insulating thin film layers. The usage of metal thin films is thus increasing rapidly in the field of magnetic recording.
When a metal thin film is used in a magnetic head, a ceramic base is usually employed as a substrate. The conventional ceramic substrate has a disadvantage in that it has a thermal expansion coefficient markedly lower than that of the metal itself and hence the metal film tends to be detached from the ceramic base during exposure to higher temperatures. For example, the coefficient of linear expansion .alpha. of "Permalloy" or "Sendust" used in thin film magnetic heads amounts to about 130 to 160.times.10.sup.-7 /.degree. C., whereas those of commercially used barium titanate and calcium titanate ceramics are considerably lower, amounting to about 90 to 100.times.10.sup.-7 /.degree. C. and 100 to 120.times.10.sup.-7 /.degree. C., respectively.
Thus, there is a need for a ceramic base having a linear thermal expansion coefficient which is comparable to that of the aforementioned metal thin films.